Remember!
What periods does the individual development of an organism consist of?
What is development and metamorphosis?
What organisms are characterized by this type of development?
Individual development of an individual, the totality of its transformations from its origin to the end of life called ontogeny. According to modern scientific concepts, the cell from which the ontogenesis of an individual begins contains specific program further development of the body. During the process of ontogenesis, this hereditary program is implemented through the interaction of the nucleus and cytoplasm of each cell, individual cells with each other, and tissues with each other. As a result of these complex relationships, based on the available genetic information and depending on external conditions, the specific individuality of the individual is formed.
In bacteria and unicellular eukaryotic organisms, ontogenesis begins at the moment of formation of the organism as a result of division of the mother cell and ends either with the death of the cell or the next division of the organism, i.e., in essence, it coincides with the cell cycle.
In multicellular organisms that reproduce asexually, ontogenesis begins with the separation of one or more cells of the maternal organism, giving rise to a new individual.
In organisms that reproduce sexually, individual development begins from the moment of fertilization and the formation of the zygote and is divided into two periods: embryonic (period of embryonic development) and postembryonic (period of post-embryonic development). The ratio of the duration of these periods in organisms different types may vary greatly.
Embryonic period (embryogenesis). This period lasts from the moment of formation of the zygote until the embryo leaves the egg or is born. It occurs in several stages (Fig. 62). In the first stage, which is called crushing, the fertilized egg divides by mitosis, resulting in 2, 4, 8, 16, etc. cells that fit tightly together. The interphase between divisions is very short, the cells do not grow, so the fragmentation process occurs very quickly. Crushing ends with the formation blastula- a hollow ball, the wall of which consists of a single layer of cells. Further, at one of the poles of the blastula, the cells begin to divide more actively and go deeper into the spherical embryo, forming an invagination. As a result of this process, a two-layer embryo is formed - gastrula. The two layers of cells that form its walls are called germ layers: outer leaf - ectoderm and internal - endoderm.
In all animals, except sponges and coelenterates, with the further development of the embryo, a third germ layer is formed between the ectoderm and endoderm - mesoderm.
Further development of the embryo is associated with the interaction of three germ layers, from which all tissues and organs of the body are formed. Development of fetal organ systems – organogenesis- occurs in a certain sequence. In chordates, it begins with the formation of the rudiment of the notochord and the nervous system. On the dorsal side of the embryo, a group of ectoderm cells separates into a long plate. These cells begin to actively divide, plunging into the body of the embryo and forming a groove, the edges of which gradually come closer together and then close, forming the primary neural tube.
Rice. 62. Crushing of the fertilized lancelet egg and formation of germ layers
In addition to the nervous system, skin glands, tooth enamel, hair, nails, and skin epithelium also arise from the ectoderm. The endoderm gives rise to the tissues lining the intestines and respiratory tract, forming the liver and pancreas. From the mesoderm, muscles, cartilage and bone skeleton, organs of the excretory, reproductive and circulatory systems of the body are formed.
During the process of embryogenesis, there is a close interaction between the parts of the developing embryo: the rudiment of one organ or organ system determines (induces) the location and time of formation of another organ or organ system.
The mutual influence of the parts of the embryo has been demonstrated in numerous experiments. German researchers Hans Spemann and Hild Mangold took from a newt embryo at the early gastrula stage a section of the dorsal side of the body, from which the notochord and mesoderm would subsequently develop, and transplanted it to the ventral side of another gastrula. As a result, an additional neural tube was formed on the ventral side of the second embryo from the cells that were supposed to give rise to the skin. This phenomenon is called embryonic induction.
Differentiation of embryonic cells does not occur immediately, but at a certain stage of development. In the early stages of cleavage, the cells of the embryo are not yet specialized, so each of them can give rise to a whole organism. If for any reason these cells separate, two identical embryos are formed, containing identical genetic information, each of which develops into a full-fledged individual. As a result, identical or monozygotic twins are born. In the human population, these are the only people who have an identical genotype and are copies of each other.
In some animals, the embryo at an early stage of development is divided into several fragments. Moreover, each of the resulting fragments gives rise to a full-fledged organism. As a result, all cubs of one generation turn out to be absolute copies of each other. This type of reproduction is typical for armadillos. Therefore, in the litter of the nine-banded armadillo tattoo there is always an even number of same-sex cubs.
Postembryonic period. This period begins with the birth of the organism and ends with its death.
There are indirect and direct types of postembryonic development.
Indirect development. The indirect, or larval, type of development is characteristic of many invertebrates and some vertebrates (fish and amphibians). This involves the birth of an individual, sometimes completely different from the adult organism. In the process of indirect development, an individual passes through one or several larval stages (tadpole in a frog, caterpillar in a butterfly) (Fig. 63). The larvae lead an independent life, actively feed, grow and develop. After a certain time, the larva turns into an adult - occurs metamorphosis, therefore, this type of development is sometimes called development with metamorphosis. During metamorphosis, larval organs are destroyed and organs characteristic of adult animals appear.
For many species, the presence of a larval stage in the development process is an opportunity for settlement and the absence of competition between individuals of different ages for habitat and food.
Direct development. This type of development is characteristic of organisms whose young are born already similar to adults. A newly hatched duckling, a born puppy or a human child differs from an adult in its smaller size, slightly different body proportions and underdevelopment of some organ systems, such as the reproductive system. Direct development is oviparous or intrauterine.
Rice. 63. Consecutive stages of metamorphosis in a frog (from bottom to top): tadpoles in eggs, the beginning of metamorphosis, a frog with the remains of a tail
Non-larval, or oviparous, the type of development is characteristic of reptiles, birds, oviparous mammals and a number of invertebrates. The eggs of these organisms are rich in nutrients (yolk), and the embryo can long time develop inside the egg.
Intrauterine This type of development is characteristic of all higher mammals, including humans. All vital functions of the embryo in this type of development are carried out through interaction with the maternal body through a special organ - the placenta.
Embryonic development ends with the process of birth. After birth, active growth of the body is usually observed, i.e. an increase in its size and weight. Most animals, as they grow older, grow more and more slowly and, having reached a certain age, stop growing. This type of growth is called certain. At uncertain growth type organisms grow throughout their lives, such as mollusks, fish and amphibians. After active growth is completed, the body enters the stage of maturity, which is associated with childbirth. The process of individual development ends with aging and death.
Review questions and assignments
1. What is called the individual development of an organism?
2. List the periods of ontogenesis.
3. Which development is called embryonic and which is called postembryonic?
4. What types of postembryonic development of the organism exist? Give examples.
5. What is the biological significance of metamorphosis?
6. Tell us about the germ layers.
7. What is cell differentiation? How is it carried out during embryonic development?
8. Describe the concept of “growth”. What is a certain height? Uncertain growth?
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ONTOGENESIS
Ontogenesis is the individual development of an organism, which is based on the implementation of hereditary information at all stages of existence in certain environmental conditions; it begins with the formation of a zygote (during sexual reproduction) and ends with death.
The ontogeny of multicellular animals that reproduce sexually is divided into two periods: embryonic (fetal) and postembryonic.
EMBRYONAL PERIOD
The embryonic period begins with the formation of the zygote and ends with the release of the egg membranes or the birth of the organism.
The embryonic development of most multicellular animals includes three main stages:
1. crushing;
2. gastrulation;
3. histo- and organogenesis.
1. Crushing
Crushing stage characterized by the formation of a multicellular single-layer embryo - the blastula stage.
The type of egg crushing depends on the amount of yolk and the nature of its distribution.
There are three main types of eggs:
- isolecithal eggs - contain little yolk, and it is evenly distributed; such eggs are found in lancelets and mammals.
- telolecithal eggs are typical for amphibians, reptiles, and birds; they contain a large amount of yolk, concentrated at one of the poles - the vegetative one. The opposite pole, containing the nucleus and cytoplasm without the yolk, is called the animal pole.
- centrolecithal eggs are characterized by the fact that the yolk is in the center of the cell, and the cytoplasm is located on the periphery (insect eggs).
CRUSHING TYPE
Full Incomplete
(the whole egg is crushed) (part of the egg is crushed)
uniform uneven discoidal
(forming blasto- (forming blastomeres (only the disc is crushed
measures equal in size), not equal in size), cytoplasm with nucleus)
characteristic of zygotes with characteristic of eggs with characteristic only of eggs
yolk - lancelet yolk (frogs) yolk - reptiles,
After fertilization occurs fragmentation of a diploid zygote - mitotic divisions without cell growth. During the crushing process, the volume of the embryo does not change, but the size of the cells decreases each time. The cells formed as a result of fragmentation of the zygote are called blastomeres.
With complete fragmentation (in the lancelet) at the stage of 32 blastomeres, the embryo has the appearance of a raspberry and is called morula (embryo has no cavity). At the stage of 64 blastomeres, a cavity is formed in it, and the blastomeres are located in one layer around it. This stage is called blastula (multicellular single-layer embryo). The cavity inside is called blastocoel - primary body cavity. All cells of the embryo have a diploid (2n) set of chromosomes.
2. Gastrulation
Gastrulation is the next stage of embryonic development - the formation of a two-layer embryo. In the lancelet, a 2-layer embryo is formed by invagination (invagination) of the blastoderm into the cavity of the blastocoel. The gastrula has two layers of cells: the outer ectoderm and the inner endoderm. They are called the first and second germ layers. The cavity is called the gastrocoel or cavity of the primary gut, and the entrance to it is the primary mouth, or blastopore. In invertebrates, the blastopore turns into the final mouth (protostomes), in deuterostomes (chordates), the anus is formed from the blastopore, and the mouth is formed on the opposite side of the body.
At the stage of two germ layers, the development of coelenterates (hydra, jellyfish) ends; in all other types of animals, a third germ layer is formed between the ecto- and endoderm - mesoderm (formed from endoderm cells).
Germ layers are separate layers of cells that occupy a separate position in the embryo, from which all organ systems subsequently develop.
3. Histo and organogenesis– the process of formation of tissues and organs is the next stage of embryonic development.
Ectoderm on the dorsal side of the embryo it bends, forming a groove, the edges of which meet. The resulting neural tube sinks under the ectoderm. The brain forms at the anterior end of the neural tube. The process of formation of an embryo with a complex of axial organs (neural tube, notochord, intestinal tube) is called neurulation, and the resulting embryo is called neurula. The processes of nerve cells of the central nervous system form peripheral nerves. In addition, integuments and their derivatives develop from the ectoderm (nails, hair, sebaceous and sweat glands, tooth enamel, sensory cells (receptors) of the analyzers, the adrenal medulla.
Endoderm, located under the neural tube, separates and forms
elastic cord – chord. The rest of the endoderm forms the epithelium
intestinal tube, digestive glands (liver, pancreas), respiratory organs.
From mesoderm All types of connective tissue develop: bones, cartilage, tendons, subcutaneous tissue, etc.), muscles, circulatory, excretory and reproductive systems.
PROVISORAL (TEMPORARY BODIES)
During embryogenesis, the necessary connection of the embryo with the environment is provided by special extra-embryonic organs that function temporarily and are called provisional. The purpose of provisional authorities is to provide vital important functions functions of the embryo under various environmental conditions.
Thus, in truly terrestrial animals (reptiles, birds, mammals), which have lost contact with the aquatic environment, the embryos develop in a special membrane filled with liquid - the amnion. Vertebrates that have an amnion are united in the group of higher vertebrates - amniotes.
Amniotes, in addition to the amnion, also have other embryonic membranes, the allantais and the yolk sac (reptiles, birds). In addition to the amnion, allantois and yolk sac, mammals also have a chorion.
1. Chorion (choroid) formed from the ectoderm of the embryo, covered with villi that grow into the mucous membrane of the uterus. Later, part of the chorion loses villi and is called smooth, and the place of greatest branching of the chorion villi, which most closely contacts the uterus, is called the baby's place, or placenta. Through the placenta, the fetus is supplied with nutrients, oxygen and is freed from waste products (CO 2, etc.), the placenta performs barrier functions, trapping many harmful substances and microorganisms, but alcohol, nicotine and some medications can pass through it.
2. Amnion - inner germinal membrane(aqueous membrane - amniotic sac). The function of its epithelium is the secretion of amniotic fluid, which determines the most important conditions development of the fetus, as well as the excretion of its metabolic products into the amniotic fluid, prevents the embryo from losing water, serves as a protective cushion and creates the opportunity for the embryo to have some mobility.
3. Yolk sac in mammals it is reduced, filled with liquid containing proteins and salts. In the early stages of development, it plays the role of a hematopoietic organ; the first blood cells and vessels of the embryo are formed from special blood islands; the germ cells of the embryo are also formed here; the yolk sac is part of the placenta. Later, the umbilical cord is formed from the yolk sac.
4. Allantois (urinary membrane) grows from the hindgut of the embryo until it comes into contact with the chorion, forming the chorioallantois structure, rich in blood vessels. The allantois, together with the yolk sac, participates in the formation of the umbilical cord.
POSTEMBRYONAL DEVELOPMENT
The postembryonic period of ontogenesis begins with the moment of birth or exit from the egg membranes and ends with the death of the organism. This period is characterized by growth and puberty. A distinction is made between direct and indirect (with metamorphosis) postembryonic development.
Postembryonic development
Direct – Indirect –
Characterized by growth, development with transformations (with metamorphosis)
and puberty
(reptiles, birds, with complete with incomplete
mammals) transformation: transformation:
Egg - egg
Larva (caterpillar) - larva
Pupa (tadpole)
Imago - adult individual
With direct development An organism similar to an adult individual is born, but differs from it only in size, underdevelopment of the genital organs, and also in body proportions. Postembryonic development, in this case, comes down to growth and puberty. Characteristic of reptiles, birds and mammals.
With indirect development(development with metamorphosis) - transformations, a larva emerges from the egg shells, differing from the adult organism (morphologically and physiologically). It has specialized larval organs, but lacks some adult organs. The larva feeds, grows, the larval organs are destroyed, and the organs of the adult animal are formed. Biological significance indirect development is that the organism at the larval stage grows and develops not due to the reserve nutrients of the egg, but due to independent nutrition. Consequently, this type of development is typical for organisms whose eggs contain a small amount of yolk (amphibians, many arthropods, etc.)
Thus, with indirect development, competition for food and habitat between adults and their offspring decreases. For example, a frog larva - a tadpole - feeds on plants, and an adult frog - insects. Also, in a number of species, such as corals, adult individuals lead an attached lifestyle; they cannot move. But their larvae are mobile, which contributes to the spread of the species.
The individual development of animals - ontogenesis - is divided into two periods: embryonic and postembryonic.
Embryonic period
The embryonic period consists of several stages.
1 ethane- crushing and beginning of development of the fertilized lancelet egg:
Fragmentation ends with the formation of a blastula.
2nd stage- formation of germ layers, that is, the formation of a gastrula, which has a two-layer structure. The outer layer is ectoderm, the inner layer is endoderm. Between them, a third layer appears - mesoderm. The ectoderm, endoderm and mesoderm are called germ layers.
3rd stage- formation of organs:
- from the ectoderm - skin, notochord, neural tube, from which the spinal cord, brain and sensory organs are formed;
- from the endoderm - the digestive canal, liver, lungs;
- from the mesoderm - skeleton, muscles, circulatory and excretory systems.
During the period of embryonic development, differentiation occurs in the structure and functions of cells and organs: histogenesis and organogenesis.
Histogenesis is a set of processes that ensure the formation, existence and renewal of tissues in the ontogenesis of multicellular organisms.
Organogenesis is the process of formation of organ rudiments and their further differentiation.
Postembryonic period
Postembryonic development can be:
- direct, in which the born descendants are in all respects similar to adult organisms (birds, mammals, humans);
- indirect (with metamorphosis), when a new organism is born in the form of a larva that undergoes a series of transformations in its development (in amphibians, insects, etc.)
There are three phases in postembryonic development: adolescence (juvenile), maturity phase and old age phase.
Embryonic development, the juvenile phase and the maturity phase in ontogenesis are of great adaptive importance and are necessary to ensure the conservation and reproduction of the species.
The life cycle is the totality of all phases of development of generations of a certain species. In prokaryotes and many single-celled organisms, the life cycle and cell cycle are the same. Simple and complex life cycles are known, simple cycles in plants occur without alternating generations, complex cycles are accompanied by alternating generations: sexual (gametophyte) and asexual (sporophyte). In animals, simple cycles are not accompanied by complex transformations (metamorphosis) or alternation of generations. Complex cycles are accompanied by alternation of sexual and asexual, sexual and parthenogenetic, dioecious and hermaphroditic generations.
How to make a crossword? Three preparatory stages
Compiling a crossword puzzle online or on paper usually comes down to the following steps:
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How to create a crossword puzzle online from a set of words and download it in Word?
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Why solving crosswords is very useful: London scientists will tell you.
Note that solving and searching for words for scanwords and chainwords is done in a similar way. If necessary, i.e. when solving a complex crossword puzzle, it is better to enter the description using keywords, describing exactly the essence of an object or phenomenon.
